Method of preparing oxide crystal film/substrate composite and solution for use therein

ABSTRACT

There is provided a process for preparing a composite material of an oxide crystal film and a substrate by forming a Y123 type oxide crystal film from a solution phase on a substrate using a liquid phase method, wherein problems such as cracking of the oxide crystal film, separation of the oxide crystal film from the substrate, and development of a reaction layer between the substrate and the solution can be minimized. The solvent for forming the solution phase uses either a BaO—CuO—BaF 2  system or a BaO—CuO—Ag—BaF 2  system, and when the substrate with a seed crystal film bonded to the surface is brought in contact with the solution to form (grow) the oxide crystal film on the substrate, the temperature of the solution is controlled to a temperature of no more than 850° C.

TECHNICAL FIELD

[0001] The present invention relates to a process for preparing acomposite material of an oxide crystal film and a substrate by growingan oxide crystal film with a Y123 type crystal structure from a solutionphase onto a substrate, using a liquid phase method. In particular, theinvention relates to a process that contributes to improvements in theproductivity of the oxide crystal film and substrate composite material,and improvements in the quality of the crystals of the oxide crystalfilm.

BACKGROUND ART

[0002] Examples of known solvents used in the preparation of an oxidecrystal film with a Y123 type crystal structure on top of a substrate,for example in a process in which a YBa₂Cu₃O_(6+d) crystal film(hereafter abbreviated as a “YBCO film”) is grown from a solution by aliquid phase method (such as a crystal pulling type method using asolution), include BaO—CuO, BaO—CuO—BaF₂, and BaO—CuO—BaF₂—Ag (see“Supercond. Sci. Technol., 13(2000)” pp 82-87, printed in the U.K.).

[0003] With conventional solvent compositions and atmosphericconditions, the minimum temperature for film formation was limited toapproximately 860° C. (see the above reference).

[0004] In film formation from a solution phase using a liquid phasemethod, typically a seed crystal film must first be formed on thesurface of the substrate. This seed crystal film acts as a seed forcrystal film growth, and also performs the function of protecting thesubstrate from the highly reactive solution. However, the inventors ofthe present invention have discovered that if the film formationtemperature is too high, the seed crystal can break down, and in severecases, the substrate being used can react with the solution, making thegrowing and preparation of a high quality crystal film on the substrateimpossible.

[0005] For example, in YBCO film formation using an Ag—Cu alloysubstrate, when the solution temperature was 920° C., a reaction layerbetween the substrate and the solution developed between the YBCO filmand the substrate, and this reaction layer caused the YBCO film to bemore prone to cracking. At solution temperatures exceeding 860° C., forexample during film formation at 880° C., almost no reaction layerdeveloped, but regions in which the YBCO film did not form wereobserved.

[0006] When a Ni substrate that had been covered with a nickel oxide(NiO) film by surface oxidation was used, film formation at a setsolution temperature of either 920° C. or 880° C. resulted in a partialformation of a reaction layer between the substrate and the solution,although this reaction layer formation was less pronounced in the filmformation at 880° C.

[0007] When an oxide substrate comprising a polycrystalline yttriastabilized zirconia (YSZ) was used, for film formation at temperaturesin the vicinity of 880° C., reaction between the substrate and thesolution resulted in film separation if the film formation process wasconducted over an extended time period. Furthermore, when Hastelloy (abrand name of Haynes Stellite Inc., a Ni based corrosion resistantalloy) with an oriented YSZ intermediate layer was used as a substrate,cracks developed in the YBCO film with a set solution temperature of860° C.

DISCLOSURE OF THE INVENTION

[0008] In view of the circumstances described above, an object of thepresent invention is to provide a process for preparing a compositematerial of an oxide crystal film and a substrate by forming a Y123 typeoxide crystal film on a substrate using a liquid phase method (liquidphase epitaxy (LPE)), wherein problems such as cracking of the oxidecrystal film, separation of the oxide crystal film from the substrate,and development of a reaction layer between the substrate and thesolution can be minimized, and to also provide a solution used in such aprocess.

[0009] A process for preparing a composite material of an oxide crystalfilm and a substrate according to the present invention achieves theabove object in the manner described below.

[0010] Namely, the present invention provides a process for preparing acomposite material of an oxide crystal film and a substrate, by bringinga substrate with a seed crystal film formed on the surface thereof intocontact with a solution comprising the elements for forming an oxidewith a Y123 type crystal structure, and growing an oxide crystal filmwith a Y123 type crystal structure on the substrate, wherein

[0011] a BaO—CuO—BaF₂ system or a BaO—CuO—Ag—BaF₂ system is used as thesolvent for forming the aforementioned solution, and the temperature ofthe solution is controlled to a temperature of no more than 850° C.

[0012] By setting the solution temperature to a value of no more than850° C., a high quality Y123 type oxide crystal film (that is unlikelyto display cracking or separation) can be prepared. In other words, incases where the oxide crystal film is a superconducting film, asuperconductor with a high critical temperature and a large criticalcurrent density can be obtained. The reasons for this finding are thatby setting the solution temperature to a lower temperature thanconventional values, reaction between the substrate and the seed crystalfilm is inhibited, separation of the seed crystal film is prevented, anda reaction layer is not formed on top of the substrate.

[0013] The Ba/Cu atom ratio in the solvent composition used in theprocess for preparing a composite material of an oxide crystal film anda substrate according to the present invention is typically within arange from approximately 35/65 to 22/78, and preferably fromapproximately 32/68 to 25/75. By ensuring that the Ba/Cu atom ratiofalls within this range, the solution temperature reduction effectdescribed above can be realized, enabling crystal growth to proceed atno more than approximately 850° C. (see example 7 and FIG. 3).Furthermore, if Ag is also added, as per the second of the two solventsdescribed above, then the temperature reduction effect is even morepronounced, and by using a Ag concentration that represents a saturatedconcentration, this Ag addition effect becomes even more significant.

[0014] Furthermore, the solution comprises the aforementioned solvent(either a BaO—CuO—BaF₂ system or a BaO—CuO—Ag—BaF₂ system), and theelements for forming the Y123 type oxide as solutes.

[0015] Furthermore, when the temperature is cooled from the crystalgrowing temperature down to room temperature, or down to a temperatureat which the crystal is used, thermal stress develops, although it isalready known that the amount of thermal stress that develops reduces asthe temperature difference is reduced. Accordingly, the presentinvention also displays a thermal stress reduction effect.

[0016] In the above process for preparing a composite material of anoxide crystal film and a substrate, the oxygen concentration in thecrystal growing atmosphere is preferably within a range fromapproximately 1×10⁻² to 5 mol %, and even more preferably fromapproximately 5×10⁻² to 2 mol %, and most preferably from 1×10⁻¹ to 1.5mol %. The lower this oxygen concentration becomes, the easier it is toachieve a temperature reduction effect (an effect in which thetemperature for the Y123 type crystal film formation can be lowered, andas a result, reaction between the solution and the substrate can beinhibited), although at concentrations less than approximately 1×10⁻²mol %, the Y123 type crystal will not grow (see example 5 and FIG. 1).

[0017] In the above process for preparing a composite material of anoxide crystal film and a substrate, the substrate preferably uses ametal substrate selected from a group consisting of Ni, Ag, Ni basedalloys, and Ag based alloys, or alternatively a composite metalsubstrate comprising a metal substrate, with a core formed from an Febased alloy or a Ni based alloy different from the metal substrateprovided therein. In addition, the use of composite substrates in whichan oxide film is formed on top of either a metal substrate or acomposite metal substrate as described above, or a semiconductorcomprising silicon and GaN, by a physical vapor deposition (PVD) method,a surface oxidation method or a chemical vapor deposition (CVD) method,using an oxide selected from a group consisting of MgO, LaAlO₃, Al₂O₃,SrTiO₃, yttria stabilized ZrO₂, CeO₂, Y₂O₃, and NiO, is particularlydesirable. Furthermore, the aforementioned substrate can also utilizenon-metallic substrates formed from MgO, LaAlO₃, Al₂O₃, SrTiO₃,Sr₂AITaO₆, (La_(1−x)Sr_(x))₂AlTaO₆ (wherein, 0≦x<1), yttria stabilizedZrO₂, CeO₂, Y₂O₃, NiO, Si₃N₄, or combinations thereof.

[0018] Ni, Ag, Ni based alloys, and Ag based alloys display excellentcorrosion resistance at high temperatures, and are ideally suited to useas substrates in film formation processes conducted at temperatures of800° C. or higher. Furthermore, Fe based alloys and Ni based alloysdisplay superior strength, and also offer excellent corrosion resistanceat high temperatures. As a result, using an Fe based alloy or a Ni basedalloy as the core makes it easier to achieve good mechanical strengthfor the metal substrate, and is advantageous in terms of increasing thesurface area or the length of a superconducting member. In addition, byforming a film of an oxide selected from a group consisting of MgO,LaAlO₃, Al₂O₃, SrTiO₃, yttria stabilized ZrO₂, CeO₂, Y₂O₃, and NiO ontop of the metal substrate or composite metal substrate, as a backingfilm for the seed crystal film, formation of the seed film becomeseasier. By so doing, reaction between the seed crystal and the substrateat high temperatures can be inhibited, formation of a YBCO film by aliquid phase method can be performed with good reliability, and theproperties of the crystal film can be improved.

[0019] If a composite substrate in which an oxide film is formed on topof a semiconductor comprising silicon or GaN by a physical vapordeposition (PVD) method or a chemical vapor deposition (CVD) method,using an oxide selected from a group consisting of MgO, LaAlO₃, Al₂O₃,SrTiO₃, yttria stabilized ZrO₂, CeO₂, Y₂O₃, and NiO, is used as thesubstrate, then formation of the seed film becomes easier. As a result,reaction between the seed crystal and the substrate at high temperaturescan be inhibited, formation of a YBCO film by a liquid phase method canbe performed with good reliability, the properties of the crystal filmcan be improved, and moreover, increases in the surface area of the Y123type crystal film become feasible, and increased performance can berealized by combining superconductivity with semiconductor devices.

[0020] If a non-metallic substrate formed from MgO, LaAlO₃, Al₂O₃,SrTiO₃, Sr₂AlTaO₆, (La₁Sr)₂AlTaO₆, yttria stabilized ZrO₂, CeO₂, Y₂O₃,NiO, Si₃N₄, or a combination thereof is used as the substrate, thenpotential advantages include reaction inhibition, easier formation ofthe seed film, increased surface area, and improved characteristics inRF applications of the superconducting film.

[0021] The aforementioned PVD method can utilize any one of laserablation, sputtering, electron beam deposition, ion beam assisteddeposition and resistance heating deposition.

[0022] The seed crystal is preferably a crystal film of: an oxide with aY123 type crystal structure containing either an oxide with acomposition represented by Ln_(1+y)Ba_(2−y)Cu₃O_(6+d) (wherein, Lnrepresents a positive ion element selected from a group consisting of Y,La, Pr, Nd, Sm, Eu, Gd, Dy, Ho, Er, Tm, Yb, and Lu, 0≦y≦1, and 0≦d≦1.5),or an oxide with a composition represented by(Y_(x1)Ca_(x2)La_(x3)Pr_(x4)Nd_(x5)Sm_(x6)Eu_(x7)Gd_(x8)Dy_(x9)Ho_(x10)Er_(x11)Tm_(x12)Yb_(x13)Lu_(x14))_(1+y)Ba_(2−y)Cu₃O_(6+d)(wherein, x1 to x14 are each at least 0, but no more than 1, x1+x2+ . .. +x14=1, and at least two of x1 to x14 are values other than 0, 0≦y≦1,and 0≦d≦1.5); an oxide with a K₂NiF₄ type structure containing an oxiderepresented by La₂CuO₄, Nd₂CuO₄, LaSrFeO₄, or LaSrGaO₄; an oxide with aperovskite type structure containing an oxide represented by NdGaO₃ orLaGaO₃; or an oxide containing Y—Ba₂Fe₂O₅, which represents a solidstate solution of a rare earth element in Ba₂Fe₂O₅.

[0023] Specific examples of the composition of Y123 type crystalstructures include (1)(Y_(x1)Ca_(x2)La_(x3)Pr_(x4)Nd_(x5)Sm_(x6)Eu_(x7)Gd_(x8)Dy_(x9)Ho_(x10)Er_(x11)Tm_(x12)Yb_(x13)Lu_(x14))_(1+y)Ba_(2−y)Cu₃O_(6+d)(wherein, x1 to x14 are each at least 0, but no more than 1, x1+x2+ . .. +x14=1, and at least two of x1 to x14 are values other than 0, 0≦y≦1,and 0≦d≦1.5) type structures, and preferably(Ln_(1−x)Ln′_(x))_(1+y)Ba_(2−y)Cu₃O_(6+d) (wherein, Ln and Ln′ aredifferent, and each represents a positive ion element selected from agroup consisting of Y, Ca, La, Pr, Nd, Sm, Eu, Gd, Dy, Ho, Er, Tm, Yb,and Lu, 0<x<1, 0≦y≦1, and 0≦d≦1.5) type structures, or

[0024] (2) Ln_(1+y)Ba_(2−y)Cu₃O_(6+d) (wherein, Ln represents a positiveion element selected from a group consisting of Y, La, Pr, Nd, Sm, Eu,Gd, Dy, Ho, Er, Tm, Yb, and Lu, 0≦y≦1, and 0≦d≦1.5) type structures.

[0025] The present invention also provides a solution for use in aprocess for preparing a composite material of an oxide crystal film anda substrate, by bringing a substrate with a seed crystal film formed onthe surface thereof into contact with a solution comprising elements forforming an oxide with a Y123 type crystal structure, and growing anoxide crystal film with a Y123 type crystal structure on the substrate,wherein the solvent used to form the solution is a BaO—CuO—BaF₂ systemor a BaO—CuO—Ag—BaF₂ system, and the Ba/Cu atom ratio of the solvent iswithin a range from approximately 35/65 to 22/78.

[0026] The meanings of the terms solvent, solute, and solution areidentical with the meanings described above in relation to the processfor preparing a composite material of an oxide crystal film and asubstrate according to the present invention.

[0027] The Ba/Cu atom ratio is typically within a range fromapproximately 35/65 to 22/78, and preferably from approximately 32/68 to25/75. By ensuring that the Ba/Cu atom ratio falls within this range,the solution temperature reduction effect described above can berealized, enabling crystal growth to proceed at no more thanapproximately 850° C. (see example 7 and FIG. 3). Furthermore, if Ag isalso added, as per the second of the two solvents described above, thenthe temperature reduction effect is even more pronounced, and by using aAg concentration that represents a saturated concentration, this Agaddition effect becomes even more significant.

[0028] Crystal growth using a solution of the present invention can beconducted at a low temperature that essentially prevents reactionbetween the crucible and the solution, meaning that changes in thesolution composition resulting from such reaction, and leakage of thesolution are effectively non-existent. Accordingly, problems such asdeterioration in the crystal quality due to changes in the solutioncomposition, or halting of the crystal growth due to solution leakage donot arise. Furthermore, the temperature difference between the crystalgrowth temperature and the external temperature decreases, makingcontrol of the growing temperature easier and suggesting furtherimprovements in crystal quality can be expected, and because the crystalgrowth occurs at a lower temperature, reductions in power consumptioncan also be expected.

[0029] In addition, because an oxide crystal with good crystallinity canbe grown over a long period, the same crucible can be used for manyrepetitions of the oxide crystal growing process.

[0030] Moreover, when the temperature is cooled from the crystal growingtemperature down to room temperature, or down to a temperature at whichthe crystal is used, thermal stress develops, although the amount ofthermal stress that develops reduces as the temperature difference isreduced, meaning the present invention also displays a thermal stressreduction effect.

[0031] The BaF₂ molar percentage in a solvent composition of the presentinvention is preferably within a range from approximately 5×10⁻² to 2mol %, and even more preferably from approximately 8×10⁻² to 1.5 mol %,and most preferably from 2×10⁻¹ to 1 mol % (see example 6 and FIG. 2).If the quantity of BaF₂ is too small, then realizing a temperaturereduction effect is difficult, whereas if the quantity of BaF₂ is toolarge, BaF₂ crystals are more likely to adhere to the Y123 type crystal.

[0032] In those cases in which the solvent composition includes Ag, aneven better temperature reduction effect can be achieved, and using asaturated Ag concentration produces the most marked Ag addition effect.

BRIEF DESCRIPTION OF THE DRAWINGS

[0033]FIG. 1 is a graph showing the relationship between oxygenconcentration and the minimum temperature at which crystal growth ispossible, for formation of a YBCO film by a liquid phase method (LPEmethod).

[0034]FIG. 2 is a graph showing the relationship between the BaF₂ molarpercentage within a solvent composition and the minimum temperature atwhich crystal growth is possible, for formation of a YBCO film by aliquid phase method (LPE method).

[0035]FIG. 3 is a graph showing the relationship between the Ba/Cu atomratio and the minimum temperature at which crystal growth is possible,for formation of a YBCO film by a liquid phase method (LPE method).

[0036]FIG. 4 is a graph showing the relationship between the height ofthe solution surface, which indicates the quantity of solution, and theretention time for the solution, for formation of a YBCO film by aliquid phase method (LPE method).

[0037]FIG. 5 is a resistance temperature curve for a YBCO film formed ona MgO substrate by an LPE method, measured using a DC four terminalmethod.

[0038]FIG. 6 is a schematic cross-sectional view showing the structureof a composite material of an oxide crystal film and a substrate, forthe case in which a YBCO film is formed at 820° C. using an oriented Nisubstrate.

[0039]FIG. 7 is a schematic cross-sectional view showing the structureof a composite material of an oxide crystal film and a substrate, forthe case in which a YBCO film is formed at 930° C. using an oriented Nisubstrate.

[0040]FIG. 8 is a schematic cross-sectional view showing the structureof a composite material of an oxide crystal film and a substrate, forthe case in which a YBCO film is formed at 820° C. using an orientedAg—Cu substrate.

[0041]FIG. 9 is a schematic cross-sectional view showing the structureof a composite material of an oxide crystal film and a substrate, forthe case in which a YBCO film is formed at 930° C. using an orientedAg—Cu substrate.

BEST MODE FOR CARRYING OUT THE INVENTION

[0042] As follows is a more detailed description of the presentinvention, based on a series of examples and comparative examples. Theyttria crucible and the magnesia container used were of the dimensionsshown below.

[0043] Yttria crucible: external diameter 60 mm, internal diameter 50mm, internal height 45 mm.

[0044] Magnesia container: external diameter 70 mm, internal diameter 62mm, internal height 50 mm.

[0045] Furthermore, YBCO crystals were used as examples of Y123 typecrystals, although the present invention is not restricted to YBCOcrystals. In the examples below, the liquid phase method employedinvolved growing the crystals using a pulling type method (specificallya rotation pulling method) from amongst the different solution basedgrowing methods, although other solution methods such as a Cyprus methodor a partially molten type method can also be used to grow crystalsusing a solution of the present invention.

COMPARATIVE EXAMPLE 1

[0046] 20 g of Y₂BaCuO₅ (hereafter abbreviated as “Y211”), and 180 g ofa powdered mixture of BaCuO₂ and CuO, in which the Ba/Cu atom ratio was3/5 (hereafter abbreviated as “Ba3/Cu5 powder”), were used as startingraw materials. These raw materials were placed in an yttria crucible.During filling of the crucible, the Y211 powder was first packed intothe bottom of the crucible, and the Ba3/Cu5 powder was then placed ontop. The filled yttria crucible was inserted into a magnesia container,placed in an atmosphere controlled electric furnace, and heated to 1000°C. The crucible was held at this temperature for 48 hours to melt theraw materials. During this 48 hour period, the Y211 and the moltenliquid reacted, forming YBCO crystals and producing sedimentation in thebottom of the crucible. At this point, the solution was a liquidcomprising YBCO as a solute and molten BaO—CuO as the solvent, and theBa/Cu atom ratio was 37.5/62.5. Subsequently, using a MgO single crystalsubstrate with a YBCO thin film formed thereon by pulsed laserdeposition as a seed crystal, this seed crystal was contacted with thesolution, and YBCO crystal growth was carried out. When the growing wasconducted in an atmosphere with an oxygen concentration of 1 mol %, theminimum temperature at which a good YBCO crystal could be grown was 930°C. At temperatures lower than 930° C., a YBCO crystal and a BaCuO₂crystal crystallized simultaneously.

[0047] When 4 g of BaF₂ and 3.1 g of CuO was added to the solution, theminimum temperature at which a good YBCO crystal could be grown in anatmosphere with an oxygen concentration of 1 mol % fell to 880° C.Furthermore, when excess Ag (25 g) was also added, the minimumtemperature for good YBCO crystal growth fell further to 860° C. Thesegrowing temperatures were the same as those mentioned in the reportdescribed above.

[0048] However, the solution reacts with the yttria crucible, causing areduction in the quantity of solution, and this reaction also causes thesolution to creep up the surface of the crucible and leak from thecrucible. As a result, crystal growth stops prematurely. In other words,crystal growth cannot be continued. The maximum crystal growing time ata temperature of 930° C. or higher was, on average, from 1 to 2 weeks,and at a temperature within a range from 860 to 930° C., wasapproximately 2 weeks.

COMPARATIVE EXAMPLE 2

[0049] 4 g of Y211, 177 g of Ba3/Cu5 powder, and 31.5 g of CuO were usedas starting raw materials. These raw materials were placed in an yttriacrucible. During filling of the crucible, the Y211 powder was firstpacked into the bottom of the crucible, and a mixture of the Ba3/Cu5powder and the CuO was then placed on top. The filled yttria cruciblewas inserted into a magnesia container, placed in an atmospherecontrolled electric furnace, and heated to 920° C. The crucible was heldat this temperature for 48 hours to melt the raw materials. During this48 hour period, the Y211 and the molten liquid reacted, forming YBCOcrystals (Y123 type crystals) and producing sedimentation in the bottomof the crucible. At this point, the solution was a liquid comprisingYBCO as a solute and molten BaO—CuO as the solvent, and the Ba/Cu atomratio was 30/70. Using a similar seed crystal to the comparative example1, crystal growth was conducted in an atmosphere with an oxygenconcentration of 1 mol %, and good YBCO crystal growth was possible attemperatures above 850° C.

EXAMPLE 1

[0050] When 3.6 g of BaF₂ and 3.95 g of CuO were added to the solutionof the comparative example 2 to produce a Ba/Cu atom ratio of 30/70, andthe atmospheric oxygen concentration was set at 1 mol %, the crystalgrowing temperature required to enable production of a good YBCO crystalwas able to be reduced to 836° C.

EXAMPLE 2

[0051] In the example 1, by adding 30 g of Ag to the solution, thecrystal growing temperature required to enable production of a good YBCOcrystal, under conditions including a saturated Ag solution and anatmospheric oxygen concentration of 1 mol %, fell to 806° C. Attemperatures below 806° C., both the targeted YBCO crystal and anunwanted BaCuO₂ crystal were grown simultaneously.

EXAMPLES 3, 4

[0052] In the procedure for preparing the solutions of the examples 1and 2, at the stage of placing the raw materials in the crucible, eitherBaF₂, or BaF₂ and Ag respectively, were weighed out to ensure thecorrect solvent composition, subsequently placed in the crucible, andthen heated to prepare the solution. Each solution was then used to growa YBCO crystal. The crystal growing temperatures required to enableproduction of a good YBCO crystal fell to the same temperatures observedin the examples 1 and 2 respectively.

EXAMPLE 5

[0053] Using a solution with a Ba/Cu atom ratio of 30/70 prepared in thesame manner as the example 2, and varying the oxygen partial pressure,the minimum temperature at which the YBCO crystal phase could be grownexclusively was investigated. The BaF₂ molar percentage within thesolvent composition was 1 mol %.

[0054] The atmospheric partial pressure of oxygen was varied by mixingnitrogen gas into the atmosphere to vary the atmospheric oxygenconcentration across a range from 10⁻² to 10 mol % (oxygen partialpressure: 10 to 10130 Pa). As is evident from the results of theseinvestigations shown in FIG. 1, it was discovered that the lower theoxygen concentration is, the lower the minimum temperature for growthbecomes, although if the oxygen concentration is less than 10⁻² mol %,the YBCO crystal did not grow.

EXAMPLE 6

[0055] Using a solution with a Ba/Cu atom ratio of 30/70 prepared in thesame manner as the example 2, and varying the BaF₂ molar percentagewithin the solvent composition, the minimum temperature at which theYBCO crystal phase could be grown exclusively was investigated. Theatmospheric oxygen concentration was set to 1 mol % (oxygen partialpressure: 1013 Pa).

[0056] The BaF₂ molar percentage within the solvent composition wasvaried within a range from 10⁻² to 5 mol %.

[0057] As is evident from the results shown in FIG. 2, at BaF₂ molarpercentage values less than 5×10⁻² mol %, the temperature reductioneffect is minimal, whereas increasing the BaF₂ molar percentage beyond 1mol % provides no further improvements in the temperature reductioneffect. Furthermore, it was also discovered that if crystal growth isperformed at temperatures less than 810° C., or crystal growth isperformed at temperatures less than 840° C. and with a BaF₂ molarpercentage value exceeding 2 mol %, then BaF₂ crystals tend to adhere tothe surface of the YBCO crystal.

EXAMPLE 7

[0058] Using a similar method to the example 2, the Ba/Cu atom ratio wasvaried, and the minimum temperature at which the YBCO crystal phasecould be grown exclusively was investigated.

[0059] The atmospheric oxygen concentration was set to 1 mol % (oxygenpartial pressure: 1013 Pa), and the BaF₂ molar percentage within thesolvent composition was set to 1 mol %. The Ba/Cu atom ratio was variedwithin a range from 50/50 to 10/90.

[0060] As is evident from the results of this investigation shown inFIG. 3, if the quantity of Ba is greater than a Ba/Cu ratio of37.5/62.5, then the minimum temperature for crystal growth exceeds 850°C. In contrast, if the quantity of Cu is greater than a Ba/Cu ratio of20/80, then once again the minimum temperature for crystal growthexceeds 850° C.

EXAMPLE 8

[0061] For the solution compositions and the atmospheric conditionsdescribed in the examples 1 and 2, the relationship between the solutionretention time and the fall in the surface of the solution wasdetermined for crystal growth at 820° C. and 850° C. respectively. Even4 weeks after commencing the tests, almost no fall in the solutionsurface level had been observed.

[0062] From the results in FIG. 4, which displays the time dependency ofvariations in the solution surface level, it is evident that thesolution underwent almost no reaction with the crucible, and sufferedalmost no leakage from the crucible.

[0063] In contrast, during crystal growth at temperatures exceeding 930°C., measurement of the fall in the solution surface level showed almostno fall in the surface level for the first 10 days, but a rapid fall inthe solution surface level thereafter, and in approximately 2 days thesolution had leaked from the crucible.

[0064] <Evaluation of Superconductivity>

[0065] Using the solution from the example 2, a resistance temperaturecurve was produced using a DC four terminal method for a crystal grownon a MgO substrate at an atmospheric temperature of 821° C., whichrepresents a temperature at which conventional YBCO growth has provedimpossible.

[0066] From the results of this investigation, which are shown in FIG.5, it is evident that for the produced YBCO crystal, the resistance fellto zero at approximately 92 K, and the transition to a superconductingstate occurred at a temperature exceeding 90 K. It is known thatcrystals which display a superconducting transition at this temperatureare YBCO, thereby confirming that the formed crystal was a YBCO crystal.

[0067] As follows is a description of examples of composite materials ofan oxide crystal film and a substrate.

[0068] <Preparation of a Seed Crystal Covered Substrate>

[0069] Using the process described below, a seed crystal film (YBCO) of50 to 800 nm was used to cover (was bonded to) a variety of metalsubstrates and oxide substrates (each comprising a composite material:see Table 1 and Table 2). Using a KrF excimer laser (wavelength 248 nm),a pulsed laser was irradiated onto a YBCO target, generating a plume. Atthe same time, a substrate facing the target was heated to 650 to 850°C. using a back heater. TABLE 1 Combinations of oxide substrates andoxide films formed thereon Film Yttria stabilized Substrate MgO LaAlO₃SrTiO₃ ZrO₂ CeO₂ Y₂O₃ NiO MgO — ◯ ◯ ◯ ◯ ◯ ◯ LaAlO₃ — Al₂O₃ ◯ SrTiO₃ ◯ —◯ Yttria — ◯ stabilized ZrO₂ NiO ◯ —

[0070] TABLE 2 Combinations of metal substrates and oxide films formedthereon Film Substrate MgO Yttria stabilized ZrO₂ CeO₂ Y₂O₃ Nickel ◯ ◯ ◯Nickel based alloy ◯ ◯ ◯ (Hastelloy) Silver Silver-copper alloy

EXAMPLE 9

[0071] Using the solution from the example 1, each of the aforementionedsubstrates with a bonded seed crystal film (Table 1) was contacted withthe solution surface, and a YBCO film was formed at 820° C. This processis the same as the so-called “top seeded growth” process or a liquidphase epitaxy (LPE) process.

[0072] For all of the substrates, a composite material of a YBCO filmand a substrate was obtained. Analysis of the cross-sections using bothelectron microscope observation and compositional analysis based onenergy dispersive X-ray analysis confirmed that for all the differentsubstrates, reaction between the substrate and the solution was eithernon-existent, or if present, was light and localized.

EXAMPLE 10

[0073] The solution prepared in the example 1 was used. Using ametalorganic chemical vapor deposition (MOCVD) method, a Y₂O₃ film wasformed on top of an oriented Ni tape substrate with a NiO film preparedby surface oxidation epitaxy (SOE), and a seed film of YBa₂Cu₃O_(7−x)(YBCO) was then formed thereon using a MOCVD method, and the resultingmaterial was used as a substrate for crystal growth.

[0074] A YBCO film was formed from the solution phase at 820° C. using aliquid phase epitaxial method. The thickness of the Y₂O₃ film and theYBCO seed film were approximately 100 nm and 500 nm respectively. Byconducting film formation for approximately 20 minutes, a YBCO film ofapproximately 1000 nm was obtained.

[0075] Measurement of the superconducting critical temperature of theYBCO film using a DC four terminal method revealed a value of 85 K.Investigation of the cross section of the produced sample using ascanning electron microscope revealed erosion of the NiO as a result ofreaction with the solution in a very few localized areas, but in mostareas the intended composite material of the YBCO film and thesubstrate, with no reaction layer, was obtained, as shown in FIG. 6.

COMPARATIVE EXAMPLE 3

[0076] In order to confirm that the reduction in the formationtemperature of the YBCO thick film is effective in reducing the reactionbetween the substrate and the solution used in the example 10, formationof the YBCO thick film was also carried out at temperatures higher than820° C., namely at 930° C. and 880° C.

[0077] In order to realize these two film formation temperatures, theYBCO thick films were prepared under the following sets of conditions.

[0078] Conditions 1: A solvent with a composition ofBaO:CuO:BaF₂=34.5:62.5:3, and saturated with Ag was used, the growingtemperature was set at 930° C., and the growing atmosphere was a normalexternal atmosphere.

[0079] Conditions 2: A solvent with a composition of BaO:CuO=3:7, andsaturated with Ag was used, the growing temperature was set at 880° C.,and the film formation atmosphere was formed from a mixed gas ofnitrogen and oxygen, with a total pressure of 101325 Pa (1 atmosphere),and an oxygen concentration of 1 mol % (partial pressure: 1013 Pa).

[0080] When the conditions 1 were used, film formation for approximately2 minutes yielded a YBCO film of approximately 3000 nm, whereas when theconditions 2 were used, film formation for approximately 2 minutesyielded a YBCO film of approximately 2000 nm. Measurement of thesuperconducting critical temperature for each of the YBCO films using aDC four terminal method revealed values of 61 K for the YBCO filmobtained under the conditions 1, and 70 K for the YBCO film obtainedunder the conditions 2. Furthermore, investigation of the cross sectionof the produced samples using a scanning electron microscope revealedthat for the sample prepared under the conditions 1, the solvent hadpenetrated beneath the YBCO film causing partial erosion of the NiO,whereas in the case of the sample prepared under the lower growingtemperature of the conditions 2, that type of solvent penetration hadbeen suppressed.

[0081] Considering this finding in combination with the results of theexample 2 suggests that the reaction suppression effect is due to thereduction in temperature of the LPE growing temperature.

EXAMPLE 11

[0082] A Ag—Cu alloy (Cu: 0.1% by weight, Ag: the balance) with arolling orientation texture was prepared as a substrate, and a pulsedlaser deposition method was then used to form a YBCO seed film ofthickness 500 nm on the surface, and the resulting structure was used asa substrate.

[0083] The same LPE film formation conditions as those of the example 5were used, namely, a solvent with a composition of BaO:CuO:BaF₂=29:70:1,and saturated with Ag was used, the growing temperature was set to 820°C., and the film formation atmosphere used a mixed gas of nitrogen andoxygen with a total pressure of 1 atmosphere and an oxygen concentrationof 1 mol %. Conducting film formation for approximately 20 minutesyielded a YBCO film of approximately 1000 nm. Measurement of thesuperconducting critical temperature of the YBCO film using a DC fourterminal method revealed a value of 90 K. Investigation of the crosssection of the produced sample using a scanning electron microscoperevealed that the intended structure comprising a composite material ofthe YBCO film and the substrate, with no reaction layer, had beenobtained, as shown in FIG. 8.

COMPARATIVE EXAMPLE 4

[0084] In order to confirm that the reduction in the formationtemperature of the YBCO thick film is effective in reducing the reactionbetween the substrate and the solution used in the example 11, formationof the YBCO thick film was also carried out at a temperature higher than820° C., namely at 930° C. The LPE film formation conditions includedusing a solvent with a composition of BaO:CuO:BaF₂=34.5:62.5:3, andsaturated with Ag, setting the growing temperature to 930° C., andsetting the growing atmosphere to the normal external atmosphere.Conducting film formation for approximately 2 minutes yielded a YBCOfilm of approximately 3000 nm.

[0085] Measurement of the superconducting critical temperature of theYBCO film using a DC four terminal method revealed a decrease inresistance, indicating a superconducting transition, at 90 K, butmeasurements performed at temperatures of 5 K and higher did not revealzero resistance. Furthermore, investigation of the cross section of theproduced sample using a scanning electron microscope revealed that thesolvent had penetrated beneath the YBCO film, forming a reaction layerwith the substrate, as shown in FIG. 9, and the YBCO film had alsodeveloped a multitude of cracks.

[0086] The results from the example 11 and the comparative example 4show that as a result of the reduction in the growing temperature in theLPE method, the amount of reaction was reduced, and the occurrence ofcracking was suppressed.

EXAMPLE 12

[0087] Using a combination of ion beam assisted deposition (IBAD) andion beam sputtering, and a combination of inclined substrate deposition(ISD) and pulsed laser deposition (PLD), nickel based alloys (Hastelloy)with an yttria stabilized zirconia film formed on the surface thereofwere prepared as substrates a and b respectively. In the case of thesubstrate a prepared using the combination of IBAD and sputtering, anadditional Y₂O₃ film was subsequently formed on top to complete thesubstrate.

[0088] Using a PLD method, a YBCO seed film with a thickness of at least500 nm was formed on each of the substrates a, b, thereby forming seedfilm bonded substrates.

[0089] Formation of a YBCO film was then conducted using the solutionfrom the example 9, with an oxygen concentration of 1 mol %, and a setsolution temperature of 820° C.

[0090] Analysis of the cross sections of the composite materials of aYBCO film and a substrate prepared using the substrates a and bconfirmed that reaction between the substrate and the solution waseither almost non-existent, or if present, was light and localized.

[0091] In the case of the substrate a, measurement of thesuperconducting critical temperature for the YBCO film using a DC fourterminal method revealed a value of 90 K for the produced YBCO film. Incurrent-voltage measurements, a sample processed into a width of 0.33 mmproduced a measured critical current of 4.5 A. This result correspondswith a value of 135 A for a 1 cm width.

[0092] To compare the above results with films formed at highertemperatures, when film formation was conducted at 850° C., cracksdeveloped in the film, when film formation was conducted at 880° C.,separation of the film was confirmed, and when film formation wasconducted at 930° C., dissolution of the Hastelloy substrate wasobserved. From these results it is clear that temperature reductionenables a reduction in the reaction between the substrate and thesolution, and a suppression of cracking.

INDUSTRIAL APPLICABILITY

[0093] A process for preparing a composite material of an oxide crystalfilm and a substrate according to the present invention is ideal forpreparing a large surface area superconducting crystal film on asubstrate, or preparing a long superconducting crystal film on a tapetype substrate, and is very useful industrially.

What is claimed is:
 1. A process for preparing a composite material ofan oxide crystal film and a substrate, by bringing a substrate with aseed crystal film formed on a surface thereof into contact with asolution comprising elements for forming an oxide with a Y123 typecrystal structure, and growing an oxide crystal film with a Y123 typecrystal structure on said substrate, wherein a BaO—CuO—BaF₂ system or aBaO—CuO—Ag—BaF₂ system is used as a solvent for forming said solution,and a temperature of said solution is controlled to a temperature of nomore than 850° C.
 2. A process for preparing a composite material of anoxide crystal film and a substrate according to claim 1, wherein a Ba/Cuatom ratio in said solvent comprising said BaO—CuO—BaF₂ system or saidBaO—CuO—Ag—BaF₂ system is within a range from 35/65 to 22/78.
 3. Aprocess for preparing a composite material of an oxide crystal film anda substrate according to claim 1, wherein an oxygen concentration in acrystal growing atmosphere is within a range from 5×10⁻² to 2 mol %. 4.A process for preparing a composite material of an oxide crystal filmand a substrate according to claim 1, wherein said substrate uses ametal substrate formed from a metal selected from a group consisting ofNi, Ag, Ni based alloys, and Ag based alloys.
 5. A process for preparinga composite material of an oxide crystal film and a substrate accordingto claim 3, wherein said substrate uses a metal substrate formed from ametal selected from a group consisting of Ni, Ag, Ni based alloys, andAg based alloys.
 6. A process for preparing a composite material of anoxide crystal film and a substrate according to claim 4, wherein saidsubstrate uses a composite metal substrate comprising said metalsubstrate, with a core formed from an Fe based alloy or a Ni based alloythat is different from a material used in forming said metal substrateprovided therein.
 7. A process for preparing a composite material of anoxide crystal film and a substrate according to claim 5, wherein saidsubstrate uses a composite metal substrate comprising said metalsubstrate, with a core formed from an Fe based alloy or a Ni based alloythat is different from a material used in forming said metal substrateprovided therein.
 8. A process for preparing a composite material of anoxide crystal film and a substrate according to claim 1, wherein saidsubstrate uses a composite substrate in which an oxide film is formed ontop of any one of: a metal selected from a group consisting of Ni, Ag,Ni based alloys, and Ag based alloys; a composite metal comprising saidmetal, with a core formed from a Ni based alloy that is different fromsaid metal or an Fe based alloy provided therein; or a semiconductorcomprising silicon and GaN, using a physical vapor deposition (PVD)method, a surface oxidation method or a chemical vapor depositionmethod, and using an oxide selected from a group consisting of MgO,LaAlO₃, Al₂O₃, SrTiO₃, yttria stabilized ZrO₂, CeO₂, Y₂O₃, and NiO isused as said substrate.
 9. A process for preparing a composite materialof an oxide crystal film and a substrate according to claim 2, whereinsaid substrate uses a composite substrate in which an oxide film isformed on top of any one of: a metal selected from a group consisting ofNi, Ag, Ni based alloys, and Ag based alloys; a composite metalcomprising said metal, with a core formed from a Ni based alloy that isdifferent from said metal or an Fe based alloy provided therein; or asemiconductor comprising silicon or GaN, using a physical vapordeposition (PVD) method, a surface oxidation method or a chemical vapordeposition method, and using an oxide selected from a group consistingof MgO, LaAlO₃, Al₂O₃, SrTiO₃, yttria stabilized ZrO₂, CeO₂, Y₂O₃, andNiO.
 10. A process for preparing a composite material of an oxidecrystal film and a substrate according to claim 1, wherein saidsubstrate uses a non-metallic substrate formed from any one of MgO,LaAlO₃, Al₂O₃, SrTiO₃, Sr₂AlTaO₆, (La_(1−x)Sr_(x))₂AlTaO₆ (wherein,0≦x<1), yttria stabilized ZrO₂, CeO₂, Y₂O₃, NiO, Si₃N₄, or a combinationthereof.
 11. A process for preparing a composite material of an oxidecrystal film and a substrate according to claim 2, wherein saidsubstrate uses a non-metallic substrate formed from any one of MgO,LaAlO₃, Al₂O₃, SrTiO₃, Sr₂AlTaO₆, (La_(1−x)Sr_(x))₂AlTaO₆ (wherein,0≦x<1), yttria stabilized ZrO₂, CeO₂, Y₂O₃, NiO, Si₃N₄, or a combinationthereof.
 12. A process for preparing a composite material of an oxidecrystal film and a substrate according to any one of claim 1 throughclaim 11, wherein a composition of said Y123 type crystal structure isrepresented by Ln_(1+y)Ba_(2−y)Cu₃O_(6+d) (wherein, Ln represents apositive ion element selected from a group consisting of Y, La, Pr, Nd,Sm, Eu, Gd, Dy, Ho, Er, Tm, Yb, and Lu, 0≦y≦1, and 0≦d≦1.5).
 13. Aprocess for preparing a composite material of an oxide crystal film anda substrate according to any one of claim 1 through claim 11, wherein acomposition of said Y123 type crystal structure is represented by:(Y_(x1)Ca_(x2)La_(x3)Pr_(x4)Nd_(x5)Sm_(x6)Eu_(x7)Gd_(x8)Dy_(x9)Ho_(x10)Er_(x11)Tm_(x12)Yb_(x13)Lu_(x14))_(1+y)Ba_(2−y)Cu₃O_(6+d)(wherein, x1 to x14 are each at least 0, but no more than 1, x1+x2+ . .. +x14=1, and at least two of x1 to x14 are values other than 0, 0≦y≦1,and 0≦d≦1.5).
 14. A process for preparing a composite material of anoxide crystal film and a substrate according to any one of claim 1through claim 11, wherein said seed crystal is a crystal film of: anoxide with a Y123 type crystal structure containing either an oxide witha composition represented by Ln_(1+y)Ba_(2−y)Cu₃O_(6+d) (wherein, Lnrepresents a positive ion element selected from a group consisting of Y,La, Pr, Nd, Sm, Eu, Gd, Dy, Ho, Er, Tm, Yb, and Lu, 0≦y≦1, and 0≦d≦1.5),or an oxide with a composition represented by(Y_(x1)Ca_(x2)La_(x3)Pr_(x4)Nd_(x5)Sm_(x6)Eu_(x7)Gd₈Dy_(x9)Ho_(x10)Er_(x11)Tm_(x12)Yb_(x13)Lu_(x14))_(1+y)Ba_(2−y)Cu₃O_(6+d)(wherein, x1 to x14 are each at least 0, but no more than 1, x1+x2+ . .. +x14=1, and at least two of x1 to x14 are values other than 0, 0≦y≦1,and 0≦d≦1.5); an oxide with a K₂NiF₄ type structure containing an oxiderepresented by La₂CuO₄, Nd₂CuO₄, LaSrFeO₄, or LaSrGaO₄; an oxide with aperovskite type structure containing an oxide represented by NdGaO₃ orLaGaO₃; or an oxide containing Y—Ba₂Fe₂O₅, which represents a solidstate solution of a rare earth element in Ba₂Fe₂O₅.
 15. A solution foruse in a process for preparing a composite material of an oxide crystalfilm and a substrate by bringing a substrate with a seed crystal filmformed on a surface thereof into contact with a solution comprisingelements for forming an oxide with a Y123 type crystal structure andgrowing an oxide crystal film with a Y123 type crystal structure on saidsubstrate, wherein a solvent used to form said solution is aBaO—CuO—BaF₂ system or a BaO—CuO—Ag—BaF₂ system, and a Ba/Cu atom ratioof said solvent is within a range from 35/65 to 22/78.
 16. A solutionfor use in a process for preparing a composite material of an oxidecrystal film and a substrate according to claim 15, wherein a BaF₂ molarpercentage in said solvent is within a range from 5×10⁻² to 2 mol %. 17.A solution for use in a process for preparing a composite material of anoxide crystal film and a substrate according to either one of claim 15and claim 16, wherein a concentration of Ag in said solvent is asaturated concentration at a film formation temperature for said oxidecrystal film.